Biocontrol potential of endosymbiotic bacteria of entomopathogenic nematodes against the tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

Background: The tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is a major pest of tomato plants threatening global tomato production. The control of the pest is becoming increasingly difficult due to the rapid development of resistance to insecticides. Entomopathogenic nematodes (EPNs) of families Steinernematidae and Heterorhabditidae are successful biocontrol agents for many insect pests. Recently, their bacteria, Xenorhabdus spp. and Photorhabdus spp. have attracted great attention due to their major role in the pathogenicity of EPNs. In the present study, the pathogenicity of eleven EPNs isolates belonging to Steinernema feltiae and Heterorhabditis bacteriophora species was screened against the 1st/2nd and 3rd/4th instar larvae of T. absoluta. The cell-free supernatants and cell suspensions of the symbiotic bacteria from the most efficient isolates were further evaluated for their biocontrol potential in the oral and contact treatments on the larvae of T. absoluta. Results: KBC-4 and MCB-8 isolates of S. feltiae showed superior virulence relative to other EPNs species/isolates and induced 90% larval mortality against the 3rd/4th instar of T. absoluta larvae, whereas there were no clear differences in the efficacy of EPNs species/isolates against the 1st/2nd instar of T. absoluta. The 1st/2nd instar of T. absoluta larvae was more susceptible to cell-free supernatants and cell suspension of selected EPNs. The highest mortality (80%) was obtained from X. bovienii MCB-8 strain in the contact treatment of supernatants. In contact treatment of cell suspension, higher mortalities were obtained compared to oral treatments. Mortality rates ranged between 30 and 57.5% in the contact treatments of cell suspensions while the highest mortality did not exceed 20% in oral treatments. The antifeedant activity was observed in oral treatments of cell suspension and most of the larvae avoided feeding on treated leaves. Conclusion: The results indicated that symbiotic bacteria of EPNs had a great potential against T. absoluta larvae and contact treatment of cell-free supernatants against early instars of T. absoluta can be an ideal application. However, further studies are needed to investigate the field effectiveness of symbiotic bacteria.